1. Field of the Invention
This invention relates to a process of producing sulfuric acid from SO.sub.2 -containing gases comprising forming SO.sub.3 by a catalytic reaction and absorbing the SO.sub.3 in concentrated sulfuric acid, in which process the SO.sub.2 -containing gas is purified and cooled, SO.sub.2 -containing gas which has been purified and cooled is predried in contact with dilute sulfuric acid, the SO.sub.2 -containing gas is finally dried with concentrated sulfuric acid, and dilute sulfuric acid which has been used for predrying is strengthened in direct contact with water-absorbing gases and is returned to the preliminary drying stage.
2. Discussion of the Prior Art
In a conventional acid system of the contact process plant, sulfuric acid is produced in that SO.sub.2 is catalytically reacted to form SO.sub.3 and the latter is absorbed in sulfuric acid. The SO.sub.2 -containing gases are dried with concentrated sulfuric acid before they are fed to the contact process plant. The drying acid absorbs the water which is contained in the SO.sub.2 -containing gases and is thus diluted. The concentration of the drying acid is maintained at a constant value since part of the drying acid is replaced by concentrated absorbing acid. To produce a product acid having a desired sulfuric acid concentration, the water balance of the system must be maintained. To that end, the amount of water absorbed by the drying acid must not exceed a certain value, which depends on the SO.sub.2 content of the gas. If the SO.sub.2 -containing gases contain more water, the surplus water content must be removed before the drying step. For this purpose the SO.sub.2 -containing gases before being dried are cooled to a temperature at which surplus water condenses and the residual water vapor content of the gas does not exceed the water balance. The SO.sub.2 -containing gases are cooled to that temperature in a conventional gas-purifying and -cooling plant, in which cooling is properly effected. The gases are cooled by a direct contact of the gases with dilute sulfuric acid and/or water or are indirectly, cooled by means of water and/or air. The cooling effect is limited by the temperature of the water or air which is available.
If the scrubbing and cooling acid is subjected to direct cooling, as is described in British Pat. No. 744,465, the cooling effect will be restricted by the temperature and the moisture content of the air. Air having a high humidity can effect only a small evaporation of water and has a correspondingly small cooling effect. In that process, the SO.sub.2 -containing hot gases are cooled and scrubbed in a first stage of the conventional gas-purifying and -cooling plant with circulating dilute sulfuric acid having a concentration of 45 percent, part or all of the used scrubbing acid is cooled by a direct contact with air, whereby water is evaporated, and the cooled scrubbing acid is recycled to the scrubber. In a second stage, the gases are cooled further by a direct contact with a scrubbing fluid which is circulated and is cooled by an indirect heat exchange. As a result, the final cooling in the second stage depends again on the temperature of the coolant. The limitation of the cooling temperature is particularly critical with gases which have a low SO.sub.2 content, e.g., below 5 percent by volume and, when oleum is to be produced, also with gases having a low SO.sub.2 content, e.g., below 6 percent by volume. The purified and cooled gases are generally treated for a removal of sulfuric acid mists, which would otherwise pass through the dryer and enter the contact process plant. This removal of sulfuric acid mists is usually effected in a mist-collecting electrostatic precipitator. As described in U.S. Pat. No. 3,684,440 and German Pat. No. 1,992,012, a coolant at high temperatures can be used to cool the gas to low temperatures in a conventional gas-purifying and cooling plant if the end gas of the contact process plant is employed. In that case the hot gases are scrubbed in a first stage with dilute sulfuric acid of 2 to 7 percent concentration and the used scrubbing acid is cooled in direct contact with the end gas with evaporation of water and is recycled to the scrubber. The end gas may be precooled by an indirect heat exchange. The SO.sub.2 -containing cooled gases are then fed in the usual manner to a mist-collecting electrostatic precipitator. That process has the disadvantage that a large part of the SO.sub.2 in solution in the highly dilute sulfuric acid is driven out by the end gas and enters the atmosphere when the SO.sub.2 content in the end gas amounts only to 500 ppm, as is generally required at the present time, rather than the stated 1500 ppm.
If the SO.sub.2 -containing gas cannot be cooled in the conventional gas-purifying and cooling plant to the low temperature required to maintain the water balance because the cooling water or cooling air is at an excessively high temperature, the remaining surplus water content of the SO.sub.2 -containing gases can be removed by a preliminary drying with dilute sulfuric acid.
It is known from U.S. Pat. No. 2,415,142 that the surplus water content of SO.sub.2 -containing gases which have been conventionally purified and cooled can be removed by a preliminary drying with dilute sulfuric acid of 35 to 85 percent weight, which is conducted in a countercurrent to the gas, whereafter the dilute sulfuric acid effluent is reheated, if desired, and is strengthened in direct countercurrent contact with the end gas of the contact plant, and the strengthened sulfuric acid is cooled and is then re-used for preliminary drying. This process can be performed in a plurality of stages but has the disadvantage that all of the dilute sulfuric acid is pumped into the strengthener. A relatively large amount of SO.sub.2 is driven off in the strengthener and enters the exhaust gas.
It is an object of the invention to remove by an economical preliminary drying any surplus water from the SO.sub.2 -containing gases which have been conventionally purified and cooled and to minimize the amount of SO.sub.2 which enters the atmosphere.